In 3.7.7 user had trouble generating PDF with the WP External Links plugin active (https://wordpress.org/plugins/wp-external-links/). Test it out and see if there is a problem.
I can confirm this behavior in 4.0.4 -
The plugin author has a FAQ where he outlines some filters and methods to alleviate.
Using wpel_apply_settings you might be able to work something in at runtime, but to ignore all links he is looking for a class or link attribute. Not sure how helpful those will be. As more time permits down the road, I might be able to revisit and try a few solutions.
Resolve conflict with WP External Links plugin
The WP External Links plugin conflicts with Gravity PDF when trying to display the PDF
This method disables the \WPEL_Front::scan() method which was causes the problem.
A patch for this will be available in Gravity PDF 4.1.